It is common practice to stimulate the production of hydrocarbon wells using fluids that are pumped at high pressures and flow rates into the production zones of the well. The stimulation fluids pumped into the production zones may be highly acidic, and may also be laden with abrasive proppants such as bauxite or silica sand. Consequently, such fluids are frequently corrosive and/or abrasive and can cause irreparable damage to wellhead equipment if they are pumped directly through the spools and valves that make up the wellhead. To prevent such damage, wellhead isolation tools have been invented and various configurations are known. Examples of such tools are taught in at least the following patents and patent applications:
U.S. Pat. No. 3,830,304--Cummins PA1 U.S. Pat. No. 4,241,786--Bullen PA1 U.S. Pat. No. 4,632,183--McLeod PA1 U.S. Pat. No. 4,111,261--Oliver PA1 U.S. Pat. No. 4,867,243--Garner et al. PA1 U.S. Pat. No. 5,372,202--Dallas PA1 U.S. Pat. No. 5,332,044--Dallas PA1 Canadian Patent 1,292,675--McLeod PA1 Canadian Patent 1,277,230--McLeod PA1 Canadian Patent 1,281,280--McLeod PA1 Canadian Patent Application 2,055,656--McLeod
All of the wellhead isolation tools described in the patents and applications listed above operate on the same general principle. Each includes a mandrel which is stroked through the various valves and spools of the wellhead to isolate those components from the elevated pressures and corrosive and/or abrasive fluids used in the production stimulation process. A top end of the mandrel is connected to one or more high pressure valves through which the stimulation fluids are pumped. A bottom end of the mandrel is provided with a packoff assembly for achieving a fluid seal with the production tubing in the well. The mandrel is stroked down through the wellhead to an extent that it enters a top of the production tubing string where the packoff assembly seals against the inside of the production tubing so that the wellhead is completely isolated from the stimulation fluids.
The internal passage through a standard wellhead valve is about 2.56" (6.5 cm). The internal diameter of a standard production tubing is about 2.441" (6.2 cm). A mandrel for a wellhead isolation tool must be constructed to withstand at least about 10,000 psi. Consequently, the maximum internal diameter for a mandrel of any one of the wellhead isolation tools described in the patents listed above is about 1.5" (3.8 cm) when designed for use with a wellhead and production tubing of standard dimensions. If stimulation fluids are pumped through a mandrel of that size at 200 feet per second, the fluid transfer rate is about 26 barrels per minute (BPM). Higher transfer rates for abrasive fluids are undesirable because they cause too much "washout," a phenomenon in which the mandrel and/or the production tubing is damaged by abrasive fluids which erode away the walls of those components and may erode completely through one or the other, which permits high pressure fluids to escape into the wellhead and/or the well casing. The maximum fluid transfer rate through a wellhead isolation tool having a packoff assembly is therefore about 26 BPM.
Wellhead isolation tools having a packoff assembly that seals with an inside of the production tubing also suffer from other drawbacks. First, because the packoff assembly is attached to the bottom end of the mandrel, it is the packoff assembly that leads the way through the valves and spools of the wellhead. The packoff assembly is, however, larger than the mandrel and has a leading edge of rubberized sealing material that seals against the inside of the production tubing. Because of its size, the packoff assembly has a tendency to catch on constrictions as it is stroked through the wellhead, especially if the mandrel is not perfectly straight. It is not uncommon, for example, for the packoff assembly to catch on the back pressure threads of the tubing hanger. When the packoff assembly catches on a constriction in the wellhead, the sealing material at the leading edge may be torn. The mandrel itself may also be bent or buckled because it is being hydraulically forced through the wellhead by an operator who cannot see its progress, and its relatively small diameter causes it to be weak. Second, all prior art mandrels include at least one joint, namely the joint between the mandrel and the packoff assembly. Joints are undesirable because they can create eddies in the production stimulation fluids which cause washout in the area of the joint. If joints in a mandrel can be eliminated, the incidence of washout is reduced.
When a well is stimulated to increase the production of hydrocarbons, the well stimulation equipment is generally rented from well service providers who furnish the equipment with a crew on an hourly basis. Since the stimulation of any given production zone requires a certain volume of fluids, it is desirable to pump the stimulation fluids at the highest possible rate in order to minimize expense. To date, the transfer rate has been limited by the internal diameter of the wellhead isolation tool mandrel. Although the internal diameter of the passage through the wellhead is a limiting factor on the size of a mandrel, it is desirable to increase the internal diameter of the mandrel within those limits to a maximum possible extent.